Drip irrigation systems usually include a continuous irrigation water supply line with separate emitter units installed on the line, or in the line, usually at regular intervals. Irrigation water flows through the supply line under pressure, and a small amount of water continuously drips out at the intervals where the drip emitter units are installed. Drip irrigation has proved highly successful in producing greater growth of vegetation for the same amount of water when compared with conventional irrigation techniques.
There is a continuing need for a low cost dripper system having reliable performance in terms of uniform flow rates and resistance to clogging at normal operating pressures of say between 10 to 15 psi. (The dripper of this invention operates in a range from 10 to 50 psi.) A single hole in the water supply line may be the cheapest of drip systems, but such an approach is not satisfactory in most cases. The hole in the pipe wall must be of minute size to produce the desired drip rate. However, the required hole size is so small that blockage is almost inevitable at a number of places along the line, even with filtering. Moreover, a minute hole limits the operating pressure in the supply line to a maximum of about 5 psi. At a higher, more desirable line pressure of at least 15 psi, the water jets or sprays through the holes in the pipe wall. By reliably running a drip irrigation system at the higher operating pressure, longer dripper lines can be used; more output, in terms of gallons of water per hour, is produced; and the system can work on undulating ground (up and down slopes) as well as on flat ground.
A large number of more sophisticated drip irrigation systems have been developed for the purpose of overcoming the problems inherent in a single hole in the wall of the irrigation line. A common and successful approach involves use of separate emitter units installed in or on the supply line. The emitter unit taps off a portion of the water flowing in the supply line and passes the water through a labyrinth or other circuitous path that produces a large pressure drop in the water and discharges it at a uniform drip rate. Generally, such pressure-reducing labyrinthine emitter units are successful because they can use a large enough hole in the supply pipe and a wide enough passage through the labyrinth to avoid clogging in most cases, and they can be used at higher line pressures.
There is a need for a drip irrigation emitter that is simple in structure so that manufacturing costs are low, while also having the capability of being assembled with reasonably low capital and labor costs. It is also desirable that the emitter be capable of use within thin-wall pipe as well as more permanent heavy-wall pipe. Orchards and vineyards, for example, commonly use permanent drip irrigation systems, whereas cheaper thin-wall pipe can be used for temporary drip irrigation sites. A low cost drip irrigation system can be particularly important because of its use in temporary irrigation sites where irrigation systems of the lowest possible cost are needed. For example, inexpensive temporary irrigation systems can be used because of harvesting techniques where crops such as cane sugar are harvested by bulldozing the entire field, including the above-surface portion of the irrigation system. Temporary irrigation systems also are used on temporary growing sites for row crops such as lettuce, tomatoes, strawberries, cotton, and flowers, for example.
The drip irrigation systems presently known are not entirely satisfactory in terms of low cost, reliability and uniform drip rates, non-interference with free flow in the dripper line, resistance to clogging, and capability of use with thin-wall pipe as well as more permanent heavy-wall pipes. One presently known drip irrigation unit is an insert-type system in which a portion of the drip irrigation unit is inserted into one end of an irrigation supply line. The end of another irrigation line is passed over the remainder of the drip irrigation unit. Flow is between the exterior labyrinth surface of the inserted unit and the interior of the irrigation pipe wall. The fit is a cold friction fit which can introduce serious quality control problems, since the annular spaces can be subject to considerable variation because of lack of uniformity commonly present in the pipe inner diameter. This unit must be used with heavy-wall pipe because internal flow pressures can cause a thin-wall pipe to expand outwardly just enough to allow the water passing through the labyrinth to skip over teeth in the labyrinth and short circuit a portion of the labyrinth which, in turn, can produce an undesired change in drip rate at the end of the labyrinth. The pipe is cut to insert the dripper. This creates a risk of separation in the field.
Another insert-type drip irrigation unit is a complex pressure-compensated unit in which the labyrinth for providing the pressure drop is bonded to the interior wall of the irrigation pipe by bonding legs. Pressure compensation is provided by a rubber diaphragm which is pressed into and blocks part of the labyrinth as pressure is increased within the pipe. This arrangement requires expensive equipment to insert the drippers during extrusion of the pipe.
Another insert-type drip irrigation unit is assembled during extrusion of the plastic irrigation pipe. The unit is made by heating the plastic pipe (from the heat of extrusion). The dripper emitter unit is then inserted into the desired position within the pipe. Heat from the molten pipe bonds the interior of the pipe to the exterior of the inserted dripper unit. Such extrusion equipment is expensive, in part, because it requires precise temperature control during assembly. That is, good adhesion must be provided between the pipe and the emitter to ensure that the dripper will function properly hydraulically. If the temperature is too high, the softened pipe wall can flow into portions of the passageway through the labyrinth. If the temperature is too low, the adhesion is poor. Inasmuch as this arrangement uses the inner wall of the supply pipe to form the outer wall of the flow passage through the dripper, the pressure inside the dripper can open the bond between the inside wall of the pipe and the outside surface of the dripper.
A limitation of such insert-type units is hydraulic interference with flow through the interior of the irrigation pipe. As a result, the possible free flow through the irrigation pipe is reduced which, in use, reduces the useful length of the dripper line. The pipe is either cut to allow insertion of the dripper after extrusion of the pipe, which creates the risk of separation in the field, or else expensive equipment is required to allow insertion of the dripper into the pipe during the extrusion process. Insertion of drippers after the extrusion process also can cause stress cracks in the polyethylene pipe.
Another drip irrigation system is a so-called clip-on bayonet system, in which a bayonet or barb on the emitter is passed through the wall of an irrigation pipe and so the emitter unit itself is mounted on the exterior of the supply pipe. Water is drawn through the bayonet into a labyrinth formed in the interior of the exterior dripper unit. The dripper unit is usually a high-profile unit which can cause hooking and entangling with weeds, grass, etc. when pulled around a field when laying the irrigation unit. All known bayonet systems are complex multi-component systems which are relatively expensive to manufacture and assemble because of material and manual assembly costs. Bayonet systems typically require a heavy-wall pipe to properly hold the bayonet in place, and the inserted bayonet interferes with the free flow through the irrigation line.
The present invention provides an irrigation system with a single-component drip emitter unit that can be constructed at a much lower cost, while overcoming disadvantages of the prior drip irrigation units described above.
In recent years there have been a number of drip irrigation systems in which the emitter includes a resiliently flexible membrane formed of a natural or synthetic elastomeric material. The flexible membrane is displaceable toward or away from the flow-restricting flow path in the dripper in response to flow pressure variations in the conduit so as to stabilize the drip rate of the emitter with respect to variations in line pressure. There are several pressure compensating drippers which are separate units and which are attached to the supply conduit by a barb, for example, the pressure compensation dripper shown in Mehoudar (U.S. Pat. No. 4,209,133). This is a complicated, four-part emitter which is both expensive to manufacture and expensive to install in the field. At the present time, it costs about U.S. $0.05 per emitter for labor to install such an emitter in the field. Furthermore, since farmers frequently wish to pull the dripper line out of the field, an emitter such as the '133 emitter will catch on stalks and pull off the line. To solve these problems, a dripper such as that shown in Mehoudar U.S. Pat. No. 4,210,287 was developed in which the emitter is inside the supply conduit. However, this is still an expensive solution to the problem because precautions must be taken to prevent the rubber diaphragm from being drawn out of the emitter by the vacuum which can result from water draining out of the system. This entails assembling the dripper out of several parts before inserting it into the supply tube. The machinery to insert parts into the center of the supply tube during extrusion is very expensive. Finally, the thicker wall section of the tube itself demonstrates the difficulty in producing consistent adhesion by means of the heat of the extruded material alone. This thicker wall section also increases material costs.
The present invention provides a pressure compensating emitter for a drip irrigation system which solves the problems described above. The dripper itself is a simple molded article made by using a two-material molding technique which is inexpensive in both material and labor costs. The result is a low cost dripper which is not only pressure compensating, but which also can be pulled out of the field and used again without the risk of losing emitters on the irrigation supply line. The emitter of the present invention also does not have barbs or other parts disturbing the water flow through the tube which results in less pressure loss and a saving of power costs. Other advantages also are provided.